Complex sentence: Amongst the Buddha's many disciples, Ānanda had the most retentive memory and most of the sutras of the "Sutta Pitaka" are attributed to his recollection of the Buddha's teachings during the First Buddhist council.
Split into simpler sentences: Amongst the Buddha's many disciples, Ānanda stood out for having the most retentive memory. Most of the sutras of the "Sutta Pitaka" are attributed to his recollection of the Buddha's teachings during the First Buddhist council.