Jundob Ibn 'Abdullah al-Bajali reported that the Messenger of Allah (SAWS) sent a group of Muslims to fight some mushrikeen (non-believers), and they met in battle. One of the mushrikeen was ambushing individual Muslims and killing them. One of the Muslims wanted to catch him out and kill him. Jundob said, "We used to think that that man was Usamah Ibn Zayd.
When he raised his sword, the mushrik said 'La ilaha illa-Allah,' (there is no god but Allah) but he (Usamah) killed him. A man came to the Prophet (SAWS) and reported to him about what had happened in the battle. When he told him about what had happened to the mushrik who said 'La ilaha illa-Allah,' the Prophet (SAWS) sent for Usamah and asked him, 'Why did you kill him?'
He said, 'O Messenger of Allah (SAWS), he had caused much grief to the Muslims, he killed so-and-so and so-and-so,' - and he named a number of people - 'I attacked him and when he saw the sword he said 'La ilaha illa-Allah.' The Messenger of Allah (SAWS) said, 'And then you killed him?' Usamah said 'Yes.' He (SAWS) said, 'What will you do when La ilaha illa-Allah comes to defend him on the Day of Resurrection?' He said, 'O Messenger of Allah (SAWS), pray for forgiveness for me.'" The Prophet (SAWS) simply said, "What will you do when La ilaha illa-Allah comes on the Day of Resurrection?; he did not say anything more than that. (Muslim)
According to a report narrated by Usamah Ibn Zayd, he said: "The Messenger of Allah (SAWS) sent us out on a military campaign and we reached Al-Haraqat near Juhaynah in the morning. During the battle I caught a man and he said, 'La ilaha illa-Allah,' but I stabbed him. Then I felt bad about that, and I mentioned it to the Prophet ((SAWS). The Messenger of Allah (SAWS) said, 'He said La ilaha illa-Allah and you killed him?" I said, 'O Messenger of Allah, he only said it because he was afraid of my weapon.'
He (SAWS) said, 'How could you know what was in his heart? How could you be sure whether he was sincere or not?' He kept repeating this until I wished that I had not become Muslim until that day (because embracing Islam wipes off all sins that came before - Translator)." (Muslim)
One issue that may be included under the heading of reminders is reminding people about the power of Allah. An example of this follows:
Muslim reported that Abu Mas'ood al-Badri said: "I was beating a servant of mine with a whip, and I heard a voice behind me saying, 'Listen Abu Mas'ood!' but I did not pay any attention to the voice because I was so angry. When the voice got nearer to me, I realized that it was the Messenger of Allah (SAWS), and he was saying, 'Listen Abu Mas'ood, listen Abu Mas'ood!' I dropped the whip from my hand (according to another report: the whip fell from my hand out of respect for him).
He (SAWS) said, 'Listen Abu Mas'ood, Allah has more power over you than you have over this servant.' I said, 'I will never hit a slave again." According to another report he said: "I said, 'O Messenger of Allah (SAWS), he is free for the sake of Allah,' He (SAWS) said, 'if you did not do this, the Fire of Hell would blow in your face, or said, the Fire would touch you."
According to another report also narrated by Muslim, "The Messenger of Allah (SAWS) said: "Certainly Allah has more power over you than you have over him.' So he freed him." (Muslim)