It is an act of the Sunnah
to raise the hands when standing to perform the third Rak`ah in the Salah which contains four or three Rak`ahs. This happens after rising from Tashahhud (testification recited in the sitting position in the second/last unit of Prayer) in the second Rak`ah.
It is related by
Al-Bukhari and others:
Ibn `Umar (may Allah be pleased with them both) used to say takbir and raise his hands and when he intended to bow down, he used to raise his hands. He used to raise his hands when he says "Allah listens to those who praise Him". When he stood up from the two Rak`ahs, he used to raise his hands.
Ibn `Umar (may Allah be pleased with them) ascribed this to the Prophet (peace be upon him).
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.