It is in Hadith an-Nabi (s):

ثَلاثُ لُقْمَاتٍ بِالْمِلْحِ قَبْلَ الطَّعَامِ وَثَلاثٌ بَعْدَ الطَّعَامِ تُصْرَفُ بِهِنَّ عَنِ ابْنِ آدَمَ اثْنَانِ وَسَبْعُونَ نَوْعًا مِنَ الْبَلاءِ ، مِنْهَا الْجُنُونُ ، وَالْجُذَامُ ، وَالْبَرَصُ

[Eating] three pinches of salt before food and three pinches after food will take away from the Son of Adam 72 types of affliction, among them are madness and leprosy. (Musannaf of Nisaburi)

From that Hadith, Awliyaullah know how to expand on it and explain it. It means not only will this cure 70 physical diseases, but 70 spiritual diseases from the 800 ‘forbidden’ as well. You will be cured from 70 bad spiritual diseases of the heart and ego just by taking a little bit of salt at the beginning and end, because salt carries some realities of the ocean and in the ocean there are things that no one knows, like in the story of Sayyidina Bayazid al-Bistami (q), when he was being stoned by the `ulama of his time because they thought he did something wrong after they called him to court and judged him as guilty. The judgement was that he must be stoned to death and after being stoned, he fell down and they thought he was dead; there was no stethoscope at that time, so after some time they threw his body in the dump.

He awoke after seven days, left the city and went towards the ocean. There he boarded a ship and they went out to sea, then they encountered a severe storm and the boat was about to sink. Their thinking in that time was not like today. They said, “It must be that someone bad came on this ship and is the cause of the storm.” They decided they must throw that person overboard in order for the storm to calm down. So Sayyidina Bayazid al-Bistami (q) thought to himself, “They are going to take an innocent person and kill him!” so he said, “Yes, I am that one,” and they took him and threw him in the ocean!

Grandshaykh (q) said that he was diving in the ocean and said, “Yaa Rabbee, I am not returning to land until I find the reality of this ocean.” He was moving with Tayy az-Zamaan and Tayy al-Makaan, the Powers of Moving by compressing space and time. He reached a place where he heard, “Huww, Huww, Huww,” and saw a huge number of people from Ummat an-Nabi (s) and on top of them was Sayyidina Baha’uddin Shah Naqshband (q).

He said, “I am not returning until I find the secret of that dhikr.” He felt Baha’uddin Shah Naqshband (q) saying to him, “Why are you in my territory?” so he withdrew. He saw Baha’uddin Shah Naqshband (q) as Allah created him, as a soul, because he had not yet come to dunya So, as the Prophet (s) said:

الارواح جنود مجندة , فما تعارف منها ائتلف , و ما تناكر منها اختلف

The souls were gathered like battalions and those who were related came together.

“Souls are gathered in battalions; those who know each other are together.” He saw that Sayyidina Baha’uddin Shah Naqshband (q) was going to bring all these people to be together on the Naqshbandi Sufi Way, which is the highest way to bring people to Realities. So salt has the reflection of that ocean of Baha’uddin Shah Naqshband (q). When you eat a little bit at the beginning and end, the bad desires will go and the good and positive blessings will come to you.