Musnad Ahmad 1156:142

Waki' narrated to us, Talhah ibn Yahya narrated to us, I heard it from Ubayd Allah ibn Abd Allah ibn Utbah, from Umm Salamah, who said: "The Prophet (peace and blessings of Allah be upon him) was occupied and could not pray the two rak'ahs after Dhuhr, so he prayed them after 'Asr."