It is related that Abd al-Rahmaan bin Zayd (rahimahullaah) said:

The first (matter of righteousness) in which a believing servant acts is his stomach. If the affair of his stomach remains upright (with istiqaamah), then his deen will remain upright (with istiqaamah). And if (the affair of) his stomach does not remain upright, then his deen will not remain upright.

Mawsoo'ah Ibn Abi al-Dunyaa (4/96) through Hayat al-Salaf of Ahmad al-Tayyar.

Note:. The istiqaamah (uprightness) of the stomach is in relation to eating what is halaal, lawful, pure, avoiding what is doubtful and eating in moderation and not in excess. Refer also to previous articles (see here, here, and here).

Translated by Abu Iyaad (@abuiyaadsp)