What is the (commentary) source for Buddha's daily schedule?

It's from Tipitaka like ruben2020 answered.

The commentary teachers are the Tipitaka Memorizer, so they concluded the Buddha's normal daily schedule like below...

sīlakkhandhavagga,aṭṭhakathā 45 paṇṇe

sātthakaṃyeva pana bhagavato kiccaṃ hoti. taṃ pañcavidhaṃ – purebhattakiccaṃ, pacchābhattakiccaṃ, purimayāmakiccaṃ, majjhimayāmakiccaṃ, pacchimayāmakiccanti.

tatridaṃ purebhattakiccaṃ –

bhagavā hi pātova uṭṭhāya upaṭṭhākānuggahatthaṃ sarīraphāsukatthañca mukhadhovanādisarīraparikammaṃ katvā yāva bhikkhācāravelā tāva vivittāsane vītināmetvā, bhikkhācāravelāyaṃ nivāsetvā kāyabandhanaṃ bandhitvā cīvaraṃ pārupitvā pattamādāya kadāci ekako, kadāci bhikkhusaṅghaparivuto, gāmaṃ vā nigamaṃ vā piṇḍāya pavisati; kadāci pakatiyā, kadāci anekehi pāṭihāriyehi vattamānehi. seyyathidaṃ, piṇḍāya pavisato lokanāthassa purato purato gantvā mudugatavātā pathaviṃ sodhenti, valāhakā udakaphusitāni muñcantā magge reṇuṃ vūpasametvā upari vitānaṃ hutvā tiṭṭhanti, apare vātā pupphāni upasaṃharitvā magge okiranti, unnatā bhūmippadesā onamanti, onatā unnamanti, pādanikkhepasamaye samāva bhūmi hoti, sukhasamphassāni padumapupphāni vā pāde sampaṭicchanti. indakhīlassa anto ṭhapitamatte dakkhiṇapāde sarīrato chabbaṇṇarasmiyo nikkhamitvā suvaṇṇarasapiñjarāni viya citrapaṭaparikkhittāni viya ca pāsādakūṭāgārādīni alaṅkarontiyo ito cito ca dhāvanti, hatthiassavihaṅgādayo sakasakaṭṭhānesu ṭhitāyeva madhurenākārena saddaṃ karonti, tathā bherivīṇādīni tūriyāni manussānañca kāyūpagāni ābharaṇāni. tena saññāṇena manussā jānanti – ‘‘ajja bhagavā idha piṇḍāya paviṭṭho’’ti. te sunivatthā supārutā gandhapupphādīni ādāya gharā nikkhamitvā antaravīthiṃ paṭipajjitvā bhagavantaṃ gandhapupphādīhi sakkaccaṃ pūjetvā vanditvā – ‘‘amhākaṃ, bhante, dasa bhikkhū, amhākaṃ vīsati, paññāsaṃ…pe.… sataṃ dethā’’ti yācitvā bhagavatopi pattaṃ gahetvā āsanaṃ paññapetvā sakkaccaṃ piṇḍapātena paṭimānenti. bhagavā katabhattakicco tesaṃ sattānaṃ cittasantānāni oloketvā tathā dhammaṃ deseti, yathā keci saraṇagamanesu patiṭṭhahanti, keci pañcasu sīlesu, keci sotāpattisakadāgāmianāgāmiphalānaṃ aññatarasmiṃ; keci pabbajitvā aggaphale arahatteti. evaṃ mahājanaṃ anuggahetvā uṭṭhāyāsanā vihāraṃ gacchati.

tattha gantvā maṇḍalamāḷe paññattavarabuddhāsane nisīdati, bhikkhūnaṃ bhattakiccapariyosānaṃ āgamayamāno. tato bhikkhūnaṃ bhattakiccapariyosāne upaṭṭhāko bhagavato nivedeti. atha bhagavā gandhakuṭiṃ pavisati. idaṃ tāva purebhattakiccaṃ.

atha bhagavā evaṃ katapurebhattakicco gandhakuṭiyā upaṭṭhāne nisīditvā pāde pakkhāletvā pādapīṭhe ṭhatvā bhikkhusaṅghaṃ ovadati – ‘‘bhikkhave, appamādena sampādetha, dullabho buddhuppādo lokasmiṃ, dullabho manussattapaṭilābho, dullabhā sampatti, dullabhā pabbajjā, dullabhaṃ saddhammassavana’’nti. tattha keci bhagavantaṃ kammaṭṭhānaṃ pucchanti. bhagavāpi tesaṃ cariyānurūpaṃ kammaṭṭhānaṃ deti. tato sabbepi bhagavantaṃ vanditvā attano attano rattiṭṭhānadivāṭṭhānāni gacchanti. keci araññaṃ, keci rukkhamūlaṃ, keci pabbatādīnaṃ aññataraṃ, keci cātumahārājikabhavanaṃ…pe.… keci vasavattibhavananti. tato bhagavā gandhakuṭiṃ pavisitvā sace ākaṅkhati, dakkhiṇena passena sato sampajāno muhuttaṃ sīhaseyyaṃ kappeti. atha samassāsitakāyo vuṭṭhahitvā dutiyabhāge lokaṃ voloketi. tatiyabhāge yaṃ gāmaṃ vā nigamaṃ vā upanissāya viharati tattha mahājano purebhattaṃ dānaṃ datvā pacchābhattaṃ sunivattho supāruto gandhapupphādīni ādāya vihāre sannipatati. tato bhagavā sampattaparisāya anurūpena pāṭihāriyena gantvā dhammasabhāyaṃ paññattavarabuddhāsane nisajja dhammaṃ deseti kālayuttaṃ samayayuttaṃ, atha kālaṃ viditvā parisaṃ uyyojeti, manussā bhagavantaṃ vanditvā pakkamanti. idaṃ pacchābhattakiccaṃ.

so evaṃ niṭṭhitapacchābhattakicco sace gattāni osiñcitukāmo hoti, buddhāsanā vuṭṭhāya nhānakoṭṭhakaṃ pavisitvā upaṭṭhākena paṭiyāditaudakena gattāni utuṃ gaṇhāpeti. upaṭṭhākopi buddhāsanaṃ ānetvā gandhakuṭipariveṇe paññapeti. bhagavā surattadupaṭṭaṃ nivāsetvā kāyabandhanaṃ bandhitvā uttarāsaṅgaṃ ekaṃsaṃ karitvā tattha gantvā nisīdati ekakova muhuttaṃ paṭisallīno, atha bhikkhū tato tato āgamma bhagavato upaṭṭhānaṃ āgacchanti.

tattha ekacce pañhaṃ pucchanti, ekacce kammaṭṭhānaṃ, ekacce dhammassavanaṃ yācanti. bhagavā tesaṃ adhippāyaṃ sampādento purimayāmaṃ vītināmeti. idaṃ purimayāmakiccaṃ.

purimayāmakiccapariyosāne pana bhikkhūsu bhagavantaṃ vanditvā pakkantesu sakaladasasahassilokadhātudevatāyo okāsaṃ labhamānā bhagavantaṃ upasaṅkamitvā pañhaṃ pucchanti, yathābhisaṅkhataṃ antamaso caturakkharampi. bhagavā tāsaṃ devatānaṃ pañhaṃ vissajjento majjhimayāmaṃ vītināmeti. idaṃ majjhimayāmakiccaṃ.

pacchimayāmaṃ pana tayo koṭṭhāse katvā purebhattato paṭṭhāya nisajjāya pīḷitassa sarīrassa kilāsubhāvamocanatthaṃ ekaṃ koṭṭhāsaṃ caṅkamena vītināmeti. dutiyakoṭṭhāse gandhakuṭiṃ pavisitvā dakkhiṇena passena sato sampajāno sīhaseyyaṃ kappeti. tatiyakoṭṭhāse paccuṭṭhāya nisīditvā purimabuddhānaṃ santike dānasīlādivasena katādhikārapuggaladassanatthaṃ buddhacakkhunā lokaṃ voloketi. idaṃ pacchimayāmakiccaṃ.

We can find mentions of morning, midday or noon and afternoon in AN 3.19.

In the same way, a mendicant who has three factors is unable to acquire more skillful qualities or to increase the skillful qualities they’ve already acquired. What three? It’s when a mendicant doesn’t carefully apply themselves to a meditation subject as a foundation of immersion in the morning, at midday, and in the afternoon.
AN 3.19

For the three watches of the night, we can find it in MN 53. The parts in square brackets are comments from the translator.

"And how is the disciple of the noble ones devoted to wakefulness? There is the case where a disciple of the noble ones, sitting & pacing back & forth, cleanses his mind of any qualities that would hold the mind in check. During the first watch of the night [dusk to 10 p.m.], sitting & pacing back & forth, he cleanses his mind of any qualities that would hold the mind in check. During the second watch of the night [10 p.m. to 2 a.m.], reclining on his right side, he takes up the lion's posture, one foot placed on top of the other, mindful, alert, with his mind set on getting up [either as soon as he awakens or at a particular time]. During the last watch of the night [2 a.m. to dawn], sitting & pacing back & forth, he cleanses his mind of any qualities that would hold the mind in check. This is how the monk is devoted to wakefulness.
MN 53