Asim bin Damrah (may Allah have mercy on him) said: I asked Sayyiduna Ali (may Allah be pleased with him) about the voluntary prayers of the Prophet (peace and blessings be upon him) during the day. He said: Which of you is able to do that? We said: We will do as much as we are able. He said: He (peace and blessings be upon him) would wait until the sun, on the eastern side, reached the position it has on the western side at the time of Asr; then he (peace and blessings be upon him) would offer two rak'ahs. Then he would wait until the sun rose high, became a full disc, and reached the position it has when coming from the east towards the west; then at the time of Zuhr he would stand and offer four rak'ahs, separating every two rak'ahs with the salam and invoking peace upon the angels brought near, the prophets, and the believers and Muslims who follow them. Then he would wait until the sun passed its zenith; then he would pray four rak'ahs before Zuhr, separating them in the same way; then he would pray the Zuhr prayer, then after it he would offer two rak'ahs; then before Asr he would offer four rak'ahs, separating them in the same way too. Then he mentioned the complete hadith. These sixteen rak'ahs were the voluntary prayers of the Prophet (peace and blessings be upon him) during the day, and he seldom persisted in them.
Only Asim bin Damrah narrated it from Ali (may Allah be pleased with him); Abdullah bin al-Mubarak used to declare him da'if (weak) and criticised his narration of this hadith. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4915