Usamah (may Allah be pleased with him) said: I struck a man with a spear while he was saying "La ilaha illa Allah (there is no god but Allah)". The Prophet (peace and blessings be upon him) said to Usamah: "What will you do about 'La ilaha illa Allah' on the Day of Resurrection?" He (peace and blessings be upon him) repeated this again and again, until I wished that I had not accepted Islam before that hour.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 389