Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) narrates that he (peace and blessings be upon him) said: "O Ali, the example of one who does not complete his prayer is like that of a pregnant woman who, when the time of her delivery draws near, miscarries: she is neither the mother of a child nor still pregnant. And the example of the one who prays is like that of a merchant: his profit is not secured for him until his capital is secured for him. Likewise, the voluntary prayers of the one who prays are not accepted until he performs the obligatory ones."
Musa bin Ubaydah is not used as evidence, and there is disagreement over him concerning its chain: Zayd bin al-Hubab and Asbat bin Muhammad narrated it in this way, while Sulayman bin Bilal narrated it from Musa bin Ubaydah, from Salih bin Suwayd, from Ali, likewise as marfu'.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4004