Musannaf Ibn Abi Shayba 11:184

Waki’ narrated to us, Hisham bin Sa’d narrated to us, from Zayd bin Aslam, from his father, who said: When Abu Ubaydah came to Al-Sham, he and his companions were besieged, and they were afflicted with severe hardship. He said: So he wrote to Umar, and Umar wrote to him: "Peace. As for what follows: There has never been a hardship except that Allah has placed an exit after it, and one difficulty will never overcome two eases." And he wrote to him: "O you who have believed, persevere and endure and remain stationed and fear Allah that you may be successful." He said: So Abu Ubaydah wrote to him: "Peace. As for what follows: Indeed, Allah, Blessed and Exalted is He, said: 'The worldly life is but amusement and diversion and adornment and boasting to one another and competition in increase of wealth and children,' to the end of the verse." He said: So Umar went out with the letter of Abu Ubaydah and read it to the people. He said: "O people of Medina, Abu Ubaydah has only written to you to rebuke you and urge you to Jihad." Zayd said: My father said: "I was standing in the market when a group of people appeared, wearing white, having emerged from the mountain pass, among them Hudhayfah bin al-Yaman, giving glad tidings to the people." He said: So I went out running until I entered upon Umar and said: "O Commander of the Faithful, receive glad tidings of Allah’s victory and the conquest." Umar said: "Allah is the Greatest! How many a fighter has Khalid bin al-Walid killed!"