I was watching a י"ט כסלו gathering and the speaker told a story about how a chossid was in distress and davened to the Rebbe [and the happy ending is that it worked out as he wanted].
The fifth of the Rambam's 13 principles of faith [the real list - not the truncated version we don't know who wrote that found it's way into the siddurim] says:
והיסוד החמישי, שהוא יתברך הוא הראוי לְעָבְדוֹ וּלְגַדְּלוֹ, ולהודיע גדולתו, ולעשות מִצְוֹתָיו. ושלא יעשה כזה למי שהוא תחתיו במציאוּת, מן המלאכים והגלגלים והיסודות ומה שהורכב מהם. לפי שכולם מֻטְבָּעִים על פעולתם: אין משפט ולא בחירה להם אלא לו לבדו השם יתברך. וכן אין ראוי לְעָבְדָם כְּדֵי לִהְיוֹתָם אמצעים לְקָרְבָה אליו. אלא אליו בלבד יְכַוְּנוּ המחשבות, ויניחו כל מה שזולתו. וזה היסוד החמישי הוא שהזהיר על עבודת אלילים, ורוב התורה מַזְהֶרֶת עליה.
The fifth principle That He is the One that is fitting to serve and to exalt and to make His greatness known and to do His commandments; and not to do this to that which is below Him in existence - from the angels and the stars and the spheres and the elements and what which is composed of them. As they are all designed and there is no judgement and no free choice in their actions - only to Him, may He be blessed. And so [too], it is not fitting to serve them in order that they be intermediaries to bring them closer to Him, but rather to Him alone should they direct their thoughts and leave everything besides Him. And this fifth principle is that idolatry (worship of others) is prohibited, and most of the Torah prohibits this.
"לעבדם" of course includes tfilla [which is called עבודה as per the gemara at the beginning of Taanis].
