Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.009.

‘Aishah reports that if the Prophet missed the four rak’at before zuhr, he would pray them afterward. This is related by at-Tirmizhi who calls it hasan ghareeb. Ibn Majah records that she said: “If the Prophet missed the four rak’at before zuhr, he would pray them following the two rak’at after zuhr.”

The preceding is concerned with making up the sunnah prayers that one is to pray before zuhr. Concerning making up the two rak’at after zuhr, we have the following reports as recorded by Ahmad.

Umm Salamah says: “The Prophet prayed zuhr and then he received some wealth and he sat to distribute it [and continued to do so] until the mu’azhzhin made the azhan for ‘asr. He prayed ‘asr and came to me, as it was my day, and he prayed two quick rak’at. I said: ‘What are those two rak’at, O Messenger of Allah? Have you been ordered to perform them?’ He said: ‘No, they are the two rak’at that I perform after zuhr but I was busy distributing this wealth until the azhan was made for ‘asr and I hated to miss them.'” This is related by al-Bukhari, Muslim, and by Abu Dawud in somewhat different wording.