Musnad Ahmad Hadith 22756

It is narrated from Ubadah bin Samit (may Allah be pleased with him) that once the Holy Prophet (ﷺ) went to visit Abdullah bin Rawahah (may Allah be pleased with him) when he was ill. The Prophet (ﷺ) had not yet left his bedside when he said: "Do you know who are the martyrs of my Ummah?" The companions (may Allah be pleased with them) replied: "The martyr is the Muslim who is killed in the battlefield." The Holy Prophet (ﷺ) said: "If it were so, then the martyrs of my Ummah would be very few. The one who is killed (unjustly) is also a martyr, the one who dies in a plague is also a martyr, and the woman who dies because of her child (i.e., the woman who dies during childbirth due to the pain of delivery) is also a martyr."