Sahih al-Bukhari 590

Abu Nu‘aim narrated to us: ‘Abd al-Wahid bin Ayman narrated to us: My father narrated to me that he heard ‘Aisha say: By the One who took him, he never abandoned those two rak‘ahs until he met Allah. He did not meet Allah until prayer had become difficult for him, and he used to perform much of his prayer sitting—meaning the two rak‘ahs after the afternoon prayer. The Prophet (peace and blessings be upon him) used to pray them, but not in the mosque, for fear of burdening his community. He liked to make things easier for them.