Musannaf Ibn Abi Shayba 29:123

Waki’ narrated to us, he said: Hisham bin Sa’d narrated to us, from Zayd bin Aslam, from his father, who said: When Abu Ubaydah came to Ash-Sham, he and his companions were besieged, and a severe hardship befell them. So ‘Umar wrote to him: "Peace be upon you. To proceed: There has never been a hardship except that Allah has placed relief after it, and a 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: And Abu Ubaydah wrote to him: "Peace be upon you. To proceed: Indeed, Allah has 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 this to allude to you and to urge you toward Jihad." Zayd said: My father said: I was standing in the market when a group of people in white garments approached, having descended from the mountain pass, among them Hudhayfah bin al-Yaman, bringing glad tidings. 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." So ‘Umar said: "Allah is the Greatest. Perhaps the one who says, 'If only Khalid bin al-Walid were [here]...'"