Ibn Mas'ud (may Allah be pleased with him) said: I asked the Prophet (peace and blessings be upon him) which are the major sins, and he (peace and blessings be upon him) said: "That you associate partners with Allah though He is your Creator." I asked: Then which? He said: "To kill your child for fear of provision."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15839