Swearing by other than Allaah, whether an angel, Prophet, Waliy,  or any created being is forbidden. This is based upon the authentic Hadith of Ibn Umar, may Allaah be pleased with him, that the Prophet (sallallaahu alaihi wa sallam) saw Umar bin Al-Khattab while he was riding with a group of horsemen and he heard Umar swearing by his father. So the Messenger of Allaah (sallallaahu alaihi wa sallam) called out to them: Verily, Allaah the Almighty and Majestic has forbidden you from swearing by your fathers. Whoever swears, he should swear by Allaah or be silent.  In another narration on the authority of him, it is reported that the Messenger of Allaah (sallallaahu alaihi wa sallam) said: Whoever swears should not swear except by Allaah.  The Quraish used to swear by their fathers and he (sallallaahu alaihi wa sallam) said (to them): Do not swear by your fathers.  These were narrated by Muslim and others. So the Prophet (sallallaahu alaihi wa sallam) prohibited swearing by other than Allaah, and a prohibition normally means Tahrim,  indeed, it has been authentically reported from him that he called it Shirk. Umar bin Al-Khattab, may Allaah be pleased with him, said that Allaahs Messenger (sallallaahu alaihi wa sallam) said: Whoever swears by anything other than Allaah, has committed Shirk.  And it has been reported on the authority of Ibn Umar, may Allaah be pleased with him, that Allaahs Messenger (sallallaahu alaihi wa sallam) said: Whoever swore by other than Allaah, he has committed Shirk or disbelief.  Scholars have interpreted this to mean minor Shirk, and they said: It is a form of disbelief that is less than the major disbelief which takes a person out of the fold of Islam, may Allaah protect us from that. But it is one of the greatest of the major sins. This is why Ibn Masud, may Allaah be pleased with him, said: That I should swear by Allaah to a lie is better than I should swear by other than Allaah to the truth. This is supported by what Abu Hurairah reported from the Messenger of Allaah (sallallaahu alaihi wa sallam), that he said: Whoever among you swore by other than Allaah, saying in his oath: By Al-Lat, should say: La ilaha illAllaah (none has the right to be worshipped except Allaah) and whoever said to his brother: Come, and I will gamble with you, he should give charity.  So he (sallallaahu alaihi wa sallam) ordered any Muslim who swore by Al-Lat to say afterwards: La ilaha illAllaah to negate the swearing by other than Allaah, the belief in Whose Complete Oneness is obligatory. This is because of the glorification of other than Allaah, which results from ascribing to another what should only be for Him - i.e. swearing by Him. As for those Hadiths which contain swearing by parents, they are from before its prohibition, in accordance with the customs of Quraish in the days of pre-Islamic ignorance.
 Waliy: A friend, meaning, of Allaah. It refers to any believer whom Allaah loves. It is also used by various deviant groups to refer to their saints.  Muslim no. 1646 and At-Tirmithi no. 1534  Muslim no. 1646  Muslim no. 1646  Tahreem: To make something Haram (absolutely and strictly forbidden)  Ahmad 1:47 and 2:34  Abu Dawud no. 3251 and At-Tirmithi no. 1535  Al-Bukhari no. 6650 and Muslim no. 1647