(a) Nafi' (may Allah have mercy on him) said that when Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) missed a rak'ah or the like, he would stand up as soon as (the imam) said the salam, and he did not wait for the imam to stand.
(b) Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said: This is the Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3622