Abd al-Rahman bin Yazid narrates from Sayyiduna Abu Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever recites the last two verses of Surah al-Baqarah at night, they will suffice him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4760
Sufyan said that Ibn Shubrumah said: I considered how much of the Qur'an would suffice a man, and I found no surah of fewer than three verses. So I said: It is not proper then to recite fewer than three verses.
(b) Abd al-Rahman bin Yazid narrates from Sayyiduna Abu Mas'ud (may Allah be pleased with him), who traces it back to the Prophet (peace and blessings be upon him): "Whoever recites two verses of Surah al-Baqarah at night, they will suffice him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4761
Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said that a man heard another man reciting "Say: He is Allah, the One" [Surah al-Ikhlas: 1], repeating it over and over. When morning came, he came to the Messenger of Allah (peace and blessings be upon him) and mentioned what he had witnessed, as though he considered it too little. Al-Qa'nabi said: He considered that surah too little. He (peace and blessings be upon him) said: "By the One in Whose hand is my soul, it is equal to a third of the Qur'an."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4762
Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) said: Qatadah bin al-Nu'man (may Allah be pleased with him) informed me that a man stood in prayer in the time of the Prophet (peace and blessings be upon him) and recited "Say: He is Allah, the One" [Surah al-Ikhlas: 1] over and over, reciting nothing besides it. The narrator said: When morning came, a man said to the Messenger of Allah (peace and blessings be upon him): "O Messenger of Allah! A man stood in prayer at night and kept repeating 'Say: He is Allah, the One. Allah, the Self-Sufficient. He neither begets nor was He begotten, and there is none comparable to Him' [Surah al-Ikhlas: 1-4], reciting nothing besides it." It was as though that man considered the surah too little. The Messenger of Allah (peace and blessings be upon him) said: "By the One in Whose hand is my soul, this surah is equal to a third of the Qur'an."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4763