It is narrated from Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) that he said: Surah al-Ahzab was equal to Surah al-Baqarah (in its length), and in it there was also (this verse of stoning): "The old man and the old woman, when they commit zina (fornication), stone them without fail." This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.