if one missed some rakat of the prayer, must he perform raf'ul yadein on the third rakat of the imaam or according to his own number of rakat?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

In prayer, one is not obliged to raise the hands when standing for the third Rak’ah, whether he comes late to prayer, or is an Imaam, or is led in prayer and he reaches the Imaam at the beginning of the prayer, because raising one’s hands is a recommended Sunnah and not an obligation, even during Takbeerat-ul-Ihraam (saying Allaahu Akbar when starting the prayer); this (raising one’s hands) is only a recommended Sunnah. So, if one says the Takbeerat-ul-Ihraam without raising the hands, his prayer is valid.

The Fiqh Encyclopedia reads: “The jurists agreed that it is a Sunnah for the praying person to raise his hands when saying Takbeerat-ul-Ihraam as Ibn ‘Umar narrated that the Prophet used to raise his hands to the  level of his shoulders when he started the prayer…and they – i.e. the scholars of the Hanafi School of jurisprudence – have declared that if someone habitually does not raise his hands in prayer when saying the Takbeerat-ul-Ihraam, he is sinful, but his sin is not for neglecting to raise the hands, but rather for neglecting a Sunnah which the Prophet did regularly throughout his life.”

For more benefit, please refer to Fatwa 93785.

If someone comes late to prayer and wants to raise his hands, he should raise them when standing up from his first Tashahhud, and this is when the Imaam stands for the third Rak’ah. Haashiyat I’aanatu At-Taalibeen, reads: “His saying: It is desirable for someone who comes late to prayer to raise his hands when the Imaam stands from his first Tashahhud so as to follow the Imaam. Following the Imaam necessitates that if the Imaam does not raise his hands, he [the person preceded in prayer] should not raise his hands as well. However, Shaykh ‘Ali Ash-Shabraamallisi quoted Ibn Hajar as saying that he should raise his hands even if his Imaam did not raise his hands, so you should be aware of this.”

Shaykh Ibn ‘Uthaymeen issued a Fatwa to this effect as he said: “He – i.e. the person who comes late to  prayer – should raise his hands because the raising of the hands when standing for the third Rak’ah is for the sake of standing from Tashahhud, and this is relevant for someone who joins the Imaam late.”

Allaah knows best.