Praise be to Allah,
Ismaa’ eel ibn Ibraheem ibn ‘Ulayyah narrated to us from Humayd [at-Taweel] from Anas ibn Maalik who said,
“[All the parts of] the prayer of Allah’s Messenger (PBUH) were evenly spaced, and so was the prayer of Abu Bakr [as-Siddeeq], But [Umar ibn al-Khattaab] made the Fajr prayer rather longer.”
(Musnad Ahmad, Vol. 3, p. 113. The isnaad of this hadeeth is Saheeh according to the conditions set by al-Bukhaaree and Muslim.)
Source:
Thulaathiyyaat Musnad Ahmad ibn Hanbal (may Allah have mercy on him)
Reports of Imaam Ahmad that have only three narrators between him and the Prophet, peace, and blessings be upon him.
Shaykh Muhammad ibn Ahmad ibn Saalim as–Saffaareenee al–Hanbalee ~(1114 –1188 AH)