Sahih al-Bukhari 1162

Hafsah (may Allah be pleased with her) informs us in this Hadīth about one of the regular practices of the Prophet (may Allah's peace and blessings be upon him). He used to pray two Rak‘ahs, which are the regular Sunnah prayer before Fajr. He did not pray more than two, as mentioned in the narration in Sahīh Muslim on the authority of Hafsah (may Allah be pleased with her) in which she said: "When the dawn broke, he would not pray except two light Rak‘ahs." 'Light' in this Hadīth means that he used to shorten the standing, bowing, and prostration to such an extent that ‘Ā'ishah used to wonder – as mentioned in Sahīh Al-Bukhāri – whether he had recited Sūrat al-Fātihah or not.

This does not mean that he used to perform them so quickly that he would neglect some of the pillars of the prayer of standing, bowing, and prostrating. Rather, the correct meaning is that he used to shorten them in comparison to the rest of the voluntary prayers, which he would usually lengthen. "After the break of dawn", i.e. as soon as dawn broke, he would hasten to perform those two Rak‘ahs before the Iqāmah (second call to prayer) of the Fajr prayer.

This means that the time for the two regular voluntary Rak‘ahs of Fajr starts at the break of dawn and lasts until the Iqāmah of the Fajr prayer.