It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "The voluntary prayers of the night and day are (to be offered) two by two (rak‘ahs)."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 4791
Hadith Gradingحکم دارالسلام:صحيح إلا أن لفظة «والنهار» فيها كلام