Abu Salamah (may Allah be pleased with him) asked Sayyidah Aishah (may Allah be pleased with her) about the two rak'ahs that the Prophet (peace and blessings be upon him) used to pray after Asr. Sayyidah Aishah (may Allah be pleased with her) said: He (peace and blessings be upon him) used to pray them before Asr, but because of being occupied or forgetting, he prayed them after Asr, and then he kept them up constantly; and whenever he prayed a prayer, he kept it up.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4396