Musnad Abu Ya'la al-Mawsili 117:98

Shayban ibn Farrukh narrated to us, Hammad narrated to us, from Thabit, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever asks Allah for martyrdom sincerely, he will be granted it, even if it does not befall him."