Musannaf Ibn Abi Shayba 3:2353

Abu al-Ahwas narrated to us, from Abu Hamza, from a freed slave of Abu Bakra, who said: Abu Bakra entered a garden and walked around in it and looked at it, and he forgot the Asr prayer until the sun inclined. When he remembered it, he performed ablution and sat down. When the sun set, he stood up and prayed the Asr, then he prayed the Maghrib.