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@Unrul3r is correct. Because the aforementioned book is almost devoid of notes, only much later an explanation appeared. It's abhidhamma material, the context of the sentence confused me.
In the Sarvāstivādin tradition since the compilation of the Dharmaskhanda, three unconditioned dharmas have been recognized: analytical cessation (pratisankhayā-nirodha), nonanalytical cessation (apratisankhayā-nirodha) and space (ākāśa). In the Theravada tradition, only one unconditioned dharma, nirvana, is recognized [...]. According to the Samaya, the Mahāsanghika, Ekavyavahārika, Lokottaravādin, and Kaukutica schools all recognized the following nine unconditioned dharmas: (1) analytical cessation (2) nonanalytical cessation (3) space (4) the realm of infinity of space (5) the realm of infinity of consciousness (6) the realm of nothingness (7) the realm of neither consciousness nor unconsciousness (8) the law of dependent origination (9) the law of the noble path [...]. The Sarvāstivādins disagreed with this position and did not recognize the existence of a principle of Dependend Origination separate from conditioned dharmas. For the Sarvāstivādins, the law of Dependend Origination itself was conditioned.
For the Mahaśiśākas, the path to nirvana was recognized as an unconditioned dharma because the practices established by the Buddha were considered to be eternal truths. The Sarvāstivādins opposed this position by maintaining that although nirvana was unconditioned, the wisdom the Buddha had realized was conditioned and not eternal.
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Hirakawa probably only meant that "classification of dharmas into conditioned and unconditioned" is found in the Agamas, not that space were considered to be unconditioned dharma "is found in the Agamas".
Conditionality of space (akasa) was controversial topic even inside of Sarvastivada. If it would be mentioned to be non-conditional anywhere inside of Agamas (i.e. in sutras, as in 'Buddha's words') it would not be that controversial subject.
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Space is one of the 24 kinds of derived-materiality (UpadayaRupa), which is one of the 2 kinds of materiality (the other is primary-materiality, BhutaRupa, which is composed of the 4 elements of earth, water, fire, and air). They're all conditioned dhammas. Only Nibbana is the unconditioned one. Further info. is available at Vism.XIV.34