In Ziyaadaat al-Rawdah (زيادات الروضة), al-Nawawi narrated from the imaam of the Haramayn from some of the scholars that the reward for an obligatory (fardh) act of worship is seventy times more than the reward for a supererogatory (nafl) act. Al-Nawawi said: they reached this conclusion from a hadith.”
The hadith in question was quoted by Imaam al-Nawawi in al-Nihaayah (نهايته). It is the marfoo’ hadith of Salmaan concerning the month of Ramadaan: “Whoever seeks to draw closer to Allah during (this month) by doing some good deeds will be like one who does an obligatory action at any other time, and one who does an obligatory action (during Ramadaan) will be like one who does seventy obligatory actions at any other time.”
This is a da’eef (weak) hadith which was narrated by Ibn Khuzaymah, but he was uncertain as to whether it was sound or not.
(al-Talkhees al-Habeer, 3/118)
What he meant (may Allah have mercy on him) by saying that Ibn Khuzaymah was uncertain as to whether it was sound or not may be understood more clearly by referring to Saheeh Ibn Khuzaymah (3/191), where he includes it under the chapter heading, باب فضائل شهر رمضان إن صح الخبر (Chapter on the virtues of the month of Ramadaan, if the report is saheeh).
And the isnaad of this hadith includes ‘Ali ibn Zayd ibn Jad’aan, who is a da’eef (weak) narrator.
And Allah knows best.