Ati asked Lahiq about the two units of prayer before sunset. He replied that Abdullah bin Zubayr used to pray them, and Muawiya sent word to him asking, "What are these two units of prayer at sunset?"The matter was traced back to Umm Salama, who said, "The Messenger of Allah ﷺ used to pray two units before the afternoon prayer, but he was once occupied and missed them, so he prayed them after the sun had set. I never saw him pray them before that, nor after."
أخبرنا عثمان بن عبد الله قال حدثنا عبيد الله بن معاذ قال أنبأنا أبي قال
سألت لاحقا عن الركعتين قبل غروب الشمس فقال كان عبد الله بن الزبير يصليهما فأرسل إليه معاوية ما هاتان الركعتان عند غروب الشمس فاضطر الحديث إلى أم سلمة فقالت أم سلمة إن رسول الله ﷺ كان يصلي ركعتين قبل العصر فشغل عنهما فركعهما حين غابت الشمس فلم أره يصليهما قبل ولا بعد
No references are available.
[Sahih] It is also in Kubra, hadith no. 1558, and this hadith has many, many chains of transmission.