Wasa'el Al-Shia- Al-Hor Al-Ameli vol.71 p.16:
From Al-Khisal as well reported from Ali (pbuh) in the Four Hundred Word Hadith that he said: "Washing hands before and after eating increases sustenance, washes away odors, and clears sight. Eating apples matures the stomach and chewing frankincense gum tightens the teeth, inhibits mucus and drives away bad breath. Eating quinces strengthens a weak heart, soothes the stomach, enhances a heart's bravery, encourages the coward and improves the child. Eating twenty one red raisins everyday on an empty stomach banishes all illnesses but the illness of death. A believer never spits towards the Qibla, yet if he did that unintentionally, he should ask Allah's forgiveness."
Qorb Al-Isnad- Al-Humairi Al-Qommi p.46:
The Messenger of Allah (pbuhp) said: "When you belch, you must not raise your belch towards the sky, and when spitting as well. Belching is one of Allah Almighty's blessings, so when you belch, thank Allah."
Al-Da'awat- Qotb Al-Isnad Al-Rawandi p.144:
Abou Abdellah (pbuh) said: "Belching is one of Allah's blessings, so when one belches, he should thank Allah and he should not belch upwards."