swear by Allah only, and swear by Allah only when you are speaking the truth
Narrated Abu Hurayrah may Allah be pleased with him:
The Prophet ( may the peace and blessings of Allah be upon him ) said: Do not swear by your fathers, or by your mothers, or by rivals to Allah; and swear by Allah only, and swear by Allah only when you are speaking the truth.
During the pre Islamic period, the Arabs used to swear by their parents and idols. The Prophet ( may the peace and blessings of Allah be upon him ) forbade that custom, because this deed entails glorifying other than Allah. This is not befitting for any one who believes in Allah, and recalls his greatness and majesty in his heart.
The one who knows Allah the Almighty by His beautiful names and attributes must love him more than he loves for father and his mother. Rather, he would love his Lord more than he loves himself. Accordingly, if he is to swear then he may swear only by the Almighty.
Whoever swears by other than Allah while believing that the one by whom he swore is like Allah, he would go out of the fold of Islam because of his deed. However, if one uttered such words out of mistake, this would not count.
As for the believer, he does not glorify other than Allah, he would not swear - when necessary - except by him. He is obedient to him and takes into consideration his right on him. He is the one in whose hand his survival lies.
The reason for swearing by other than Allah is that one swears too much on the important and the unimportant matters, he does not care about the prohibition of that matter in the words of the Almighty ( interpretation of the meaning ) : ( And do not make [your oath by] Allah an excuse against being righteous and fearing Allah and making peace among people. And Allah is Hearing and Knowing. )
Swearing so much causes one to lose his credibility in some cases, and makes him careless in many things that should not be tolerated, such as the testimony, and the fulfillment of the promise, and so on.
A believer should be honest in his words, actions, and all his circumstances - honest with Allah, honest with people, and honest with himself.